1. Field of the Invention
This invention relates to an organic rust-proof treated copper foil and, more particularly, it relates to such a copper foil which can prevent a dust composed mainly of powdered epoxy resin from adhering to the copper foil at the time of adhering the copper foil to a substrate and is useful in the preparation of a printed wiring board.
2. Prior Art
For manufacturing a printed wiring board, a copper foil and a substrate such as an epoxy resin-impregnated glass fiber substrate are bonded together by applying heat and pressure and then the portions of the copper foil that are not necessary for making a circuit are removed by means of an acidic or alkaline etching solution. After forming the circuit and arranging additional electronic components on the circuit board, the board is dipped in a solder bath for securing the components to the board.
A copper foil to be used as a material for a circuit board has a zinc, zinc alloy or other metallic rust-proof layer formed thereon by electroplating and is subjected to surface treatments such as chromate treatment aid silane coupling agent treatment in order to meet the requirements for the circuit board, specially to prevent the rusting and the discoloration of the copper foil which may be caused due to heat at the time of adhesion of the foil to a substrate, after which the copper foil so treated is adhered to the substrate such as all epoxy resin-impreginated glass fiber substrate and then put to practice. A copper foil herein used is normally all electrodeposited one which has a matte (rough) side or surface to be generally adhered to a substrate and a glossy (smooth) side that is generally not adhered to the substrate.
The side of the copper foil not adhered to a substrate (such a side being hereinafter referred to as a "non-adhesive side") is required to have satisfactory heat oxidation resistance, solder wettability, resist ink receptability, etching susceptibility and chemical polishability.
In order to adhere a copper foil to a substrate under heat and pressure as described above, a copper foil and a substrate are put one upon another to form a laminate, after which such laminates are put one upon another with a stainless steel plate inserted as a separator between any adjecent two laminates and the whole is then pressed at a predetermined pressure and temperature thereby to adhere the copper foil to the substrate in each of the laminates. If, at this time, fine particles of an epoxy resin coming out of the substrate and a dust in the air are adhered to the non-adhesive side, they will firmly be adhered to the surface of the copper foil thereby to hinder etching during the formation of conductor circuits and cause a short circuit. This has been a serious problem. The above phenomenon is generally referred to as "resin spots". Thus, the non-adhesive side of the copper foil is specially required to have no resin spots thereon in addition to the previously mentioned requirements for the copper foil.
A well known method of organic rust-proof treatment of copper and copper alloys involves the use of benzotriazole (BTA) or a derivative thereof. BTA is also used for ten temporary organic rust-proof treatment of copper foils when they are stored before use.
Like BTA, tolyltriazole (TTA) is also used as an anti-discoloration and rust-proof agent for copper and copper alloys. See the February 1993 issue of "BOSEI KANRI (rust-proof Management)", p. 1.
In recent years, the use of carboxybenzotriazole (the November 1993 issue of "BOSEI KANRI", p. 11) and that of 3-amino-1,2,4-triazole and other BTA derivatives ("BOSEI KANRI"; November 1993, p.6) have been proposed and studied as heat-resistive organic rust-proof agents.
There are known methods for treating the surface of copper foils with BTA or a BTA derivative in order to improve the glossy side of the copper foils in long-term preservability (moisture resistance), wettability (or compatibility) with solder and resist receptability. (See, for example, Japanese Patent Pat. Appln. Nos. Hei 6-85417 and Hei 6-85455 or Nos. 85417/94 and 85455) However, it is not known at all to date to eliminate the formation of resin spots on copper foils by using a mixture of a heat-resistant BTA derivative and aminotriazole in stead of using BFA or a BTA derivative alone.